The present invention relates generally to amplifier circuits, and, more particularly, to a method and apparatus for controlling common-mode output voltage in fully differential amplifiers.
Differential signaling has been commonly used in audio, data transmission and telephone systems for many years because of its inherent resistance to external noise sources. More recently, differential signaling has become popular in high-speed data acquisition wherein, for example, differential amplifiers are used to drive inputs of analog to digital converters. In particular, a fully differential amplifier is a differential amplifier that, in addition to differential inputs, also includes differential outputs (as opposed to a single-ended output of standard operational amplifier). For example, the input and output differential signals may be voltage signals centered about (VCC−VSS)/2, within the range (VSS, VCC), wherein VSS is a substrate voltage and VCC is a power rail (or core) voltage. Such devices provide increased immunity to external common-mode noise, reduced even-order harmonics, and twice the output swing for a given voltage limit as compared to single-ended systems.
With a fully differential amplifier, a common-mode feedback loop is used to set the common-mode voltage at the output of the amplifier. One technique for achieving this is to add auxiliary current sources to the active load in the differential amplifier and to control the gates/bases of the auxiliary sources with a feedback loop that adjusts the voltage on the gates/bases until the common-mode voltage at the output of the amplifier matches a reference input to the feedback loop. This in turn results in the use of additional current source devices that increase area and capacitive loading on the output nodes of the fully differential amplifier.
Accordingly, it would be desirable to be able to implement a means for controlling the common-mode voltage in a fully differential amplifier structure without utilizing auxiliary current sources in the common-mode feedback loop, thereby saving device real estate.